E. coli bacteria live naturally in the human gut. For almost 25 years, researchers at Michigan State University in East Lansing have been growing the germs in 12 glass flasks. The microbes have been growing, and reproducing — and growing some more. Meanwhile, biologists have been watching, watching, watching.

Richard Lenski and his coworkers recently reported a new finding from the long-term project. In one flask, the bacteria the scientists were nurturing did something momentous: They gr...